Generally, cutting tools and sliding members, which are required to have good wear resistance and sliding characteristics, are used in the form having a hard coat film of titanium nitride, titanium aluminum nitride or the like as formed on the surface of a substrate such as a high speed steel or cemented carbide by the physical vapor deposition method (hereinafter referred to as “PVD method”) or chemical vapor deposition method (hereinafter referred to as “CVD method”), among others. For use as cutting tools, in particular, the hard coat film mentioned above is required to have wear resistance and heat resistance (antioxidant properties at elevated temperatures), and the above-mentioned titanium aluminum nitride (TiAlN), for instance, which can retain both the characteristics mentioned above stably up to such a high temperature as about 800° C., has recently been in wide use as a coating/covering material for cemented carbide tools and the like the tip temperature of which becomes high on the occasion of cutting.
Meanwhile, the tip of a cutting tool or the like may reach such a high temperature as 1,000° C. or above in the step of cutting. In such circumstances, it is impossible to secure a sufficient level of heat resistance with the above-mentioned hard coat film alone and, therefore, an alumina layer, for example, is further formed after the formation of the above-mentioned hard coat film to secure heat resistance, as disclosed in Japanese Patent No. 2742049.
The crystal structure of alumina varies depending on the formation temperature. When the substrate temperature is not higher than about 500° C., an amorphous structure predominates and, when it is within the range of about 500-1,000° C., the γ crystal structure predominates. Either crystal structure is in a thermally metastable state. However, when the tip temperature varies markedly in a wide range of from ordinary temperature to 1,000° C. or above in the step of cutting, as found with cutting tools, the crystal structure of alumina changes markedly, which causes problems, for example film cracking or/and peeling.
On the contrary, the α crystal structure (corundum structure), which is formed at a substrate temperature elevated to 1,000° C. or above by employing the CVD method, once formed, preserves its thermally stable structure thereafter, irrespective of temperature. Therefore, coating with α crystal structure alumina is regarded as a very effective means for providing cutting tools or the like with heat resistance.
Since, however, α crystal structure alumina cannot be formed without heating the substrate to 1,000° C. or above, as mentioned above, there is a restriction as to applicable substrates. When exposed to elevated temperatures exceeding 1,000° C., some substrates will be softened and lose their aptitude for use as substrates for wear-resistant members. Even such substrates for high temperature use as cemented carbides, when exposed to such high temperatures, cause problems such as deformation. Furthermore, the temperature range for the practical use of hard coat films, such as TiAlN films, formed as wear resistance-providing films on substrates is generally about 800° C. at the highest, so that when heated to high temperatures exceeding 1,000° C., the films may undergo denaturation, possibly leading to a deterioration in wear resistance.
To cope with such problems, a method has been proposed by which α crystal structure alumina films can be formed while still lowering the substrate temperature. For example, it is reported by O. Zywitzki, G. Hoetzsch et al. that when reactive sputtering (pulsed magnetron sputtering) is carried out using a pulsed high output power source (11-17 kW), aluminum oxide films with the corundum structure (α crystal structure) are formed even at 750° C. (cf. Surf. Coat. Technol. 86-87, 1996, pp. 640-647)
Further, in JP-A-2002-53946, it is disclosed that the method comprising forming an α crystal structure alumina coat film on an undercoat layer of a corundum structure (α crystal structure) oxide with a lattice parameter of not less than 4.779 Å but not exceeding 5.000 Å and a film thickness of at least 0.005 μm is effective.
Meanwhile, the PVD method can readily form various compound layers under milder conditions as compared with the CVD method and, among others, the sputtering technique which comprises using a metal target as the sputtering evaporation source and forming a metal compound on a substrate in a reactive gas atmosphere is in wide use since it can form various kinds of compound layers more easily. In forming alumina coat films, sputtering is carried out in an atmosphere of oxygen, which is a reactive gas, using an aluminum metal target to form alumina films on substrates.
As regards the discharge condition during sputtering in such a film-forming step, the relation between the rate of flow of oxygen gas introduced and the discharge voltage is represented by such a hysteresis curve as schematically shown in FIG. 1 when the discharge power is constant. More specifically, when the oxygen flow rate is gradually increased from a low level, the discharge voltage rapidly decreases at a certain oxygen flow rate and, conversely, when the oxygen flow rate is gradually decreased from a high level, the discharge voltage rapidly increases at a certain oxygen flow rate, as shown in FIG. 1.
Thus, such discharge conditions mentioned above are generally classifiable into three modes, as schematically illustrated in FIG. 1, namely the metal mode in which the discharge voltage is relatively high and the oxygen gas introduced is mostly consumed in alumina formation as a result of reaction with aluminum atoms formed by sputtering, the poisoning mode in which the discharge voltage is relatively low and the oxygen gas introduced still remains in excess after reaction with aluminum atoms formed by sputtering, hence the aluminum target surface, too, is oxidized, and the transition mode in which the discharge voltage shows an intermediate value between the above two discharge conditions.
When alumina films are formed in the respective discharge conditions, the rate of film formation is rapid but films containing metallic Al with a higher Al content as compared with the atomic ratio (Al:O=2:3) in alumina are formed in the metal mode discharge condition. In the poisoning mode discharge condition, the films formed contain no metallic Al and are substantially composed of alumina alone but the film formation rate becomes extremely slow since the aluminum metal target itself is also oxidized, as mentioned above, hence the quantity of Al evaporated is small.
Therefore, attempts have been made to form alumina-based films low in metallic Al content efficiently at a high film formation rate in the transition mode discharge condition by combining the respective advantages of the metal mode and poisoning mode.
In the transition mode, however, a slight change in the oxygen flow rate, which is one of the factors to be controlled, results in an abrupt substantial change to the metal mode side or poisoning mode side, as shown in FIG. 1, so that a stable discharge condition cannot be maintained. Therefore, such methods as described below have been so far proposed to secure the transition mode stably.
One method comprises maintaining the oxygen flow rate at a substantially constant level and controlling the discharge voltage. FIG. 2 shows the relation between the discharge voltage and discharge current as found in sputtering of an aluminum metal target while varying the voltage in an Ar gas and an oxygen gas (the rates of flow of both being constant). In this case, too, there are the above-mentioned three patterns of discharge condition (metal mode, transition mode, and poisoning mode), as schematically illustrated in FIG. 2. Unlike the case of FIG. 1, however, it can be seen that the transition mode condition can be maintained almost stably by adequately controlling the discharge voltage.
As for another method of stably maintaining the transition mode, JP-A-H04-325680 discloses that, when the dual magnetron sputtering (DMS) technique is employed for film formation, the discharge condition can be adjusted to the transition mode by controlling the oxygen gas flow rate so that the measured voltage of the sputtering cathode may be equal to the desired voltage. Further, in JP-A-H04-136165, it is disclosed that the discharge condition can be stabilized and qualitatively stable films can be obtained by controlling the partial pressure of a reactive gas in the film-forming chamber, for example the partial pressure of oxygen.
However, even when such a discharge condition suited for alumina film formation can be secured, it is difficult to form α crystal structure-based alumina; it is impossible to avoid contamination with γ crystal structure alumina. In particular, when the film formation rate is increased to thereby secure a desired level of productivity, or when film formation is carried out in a relatively low temperature range so that the characteristics of the substrate, for instance, may not be deteriorated, there is observed a tendency toward ready formation of γ crystal structure alumina; further investigations are required for obtaining α crystal structure-based alumina films.
Meanwhile, within the range of conditions favorable for the formation of α crystal structure alumina, it is difficult to obtain films with high hardness. On the other hand, when, for example, a bias voltage is applied to obtain alumina films higher in hardness, a mixed phase composed of α crystal structure alumina and γ crystal structure alumina will result, hence α crystal structure-based alumina films cannot be obtained. Therefore, for obtaining α crystal structure alumina films higher in film hardness, further investigations are required.
The present invention has been made in view of such circumstances as discussed above, and it is an object of the invention to provide a method of producing useful α crystal structure-based alumina films by which α crystal structure-based alumina films having good heat resistance can be efficiently formed on substrates or hard coat films such as the above-mentioned TiAlN in a relatively low temperature range in which the substrates and apparatus, among others, will not be put under thermal load and by which α crystal structure-based alumina films higher in hardness can be formed on such substrates or hard coat films.